BAGHDAD — Suspected Sunni insurgents penetrated the Baghdad security net Wednesday, hitting Shiite targets with four bomb attacks that killed at least 183 people — the bloodiest day since the U.S. troop surge began nine weeks ago.

Nationwide, the number of people killed or found dead on Wednesday was 233, which equaled the highest death toll since The Associated Press began recording daily nationwide deaths in May 2005.

Late Wednesday, Prime Minister Nouri al-Maliki ordered the arrest of the Iraqi army colonel who was in charge of security in the area around the Sadriyah market where at least 127 people died and 148 were wounded in one of the Baghdad bombings.

It was the second massive blast at the market since Feb. 3.

Defense Secretary Robert Gates called the bombings “horrifying” and accused al-Qaida of being behind them.

In the deadliest of the attacks, a parked car bomb detonated in a crowd of workers at the Sadriyah market in central Baghdad, said Raad Muhsin, an official at Al-Kindi Hospital where the victims were taken.

Among the dead were several construction workers who had been rebuilding the mostly Shiite marketplace after a bombing destroyed many shops and killed 137 people there in February, the police official said.

The laborers typically finish work around 4 p.m. each day. One of those wounded, 28-year-old Salih Mustafa, said he was waiting for a minibus to head home when the blast went off at 4:05 p.m.

“I rushed with others to give a hand and help the victims,” he said. “I saw three bodies in a wooden cart, and civilian cars were helping to transfer the victims. It was really a horrible scene.”

The market is situated on a side street lined with shops and vendors selling produce, meat and other staples. It is also about 500 yards from a Sunni shrine.

41 die in police checkpoint blast
About an hour earlier, a suicide car bomber crashed into an Iraqi police checkpoint at an entrance to Sadr City, the capital’s biggest Shiite Muslim neighborhood and a stronghold for the militia led by radical anti-U.S. cleric Muqtada al-Sadr.

The explosion killed at least 41 people, including five Iraqi security officers, and wounded 76, police and hospital officials said.

Black smoke billowed from a jumble of at least eight incinerated vehicles that were in a jam of cars stopped at the checkpoint. Bystanders scrambled over twisted metal to drag victims from the smoldering wreckage as Iraqi guards staggered around stunned.

Earlier, a parked car exploded near a private hospital in the central neighborhood of Karradah, killing 11 people and wounding 13, police said. The blast damaged the Abdul-Majid hospital and other nearby buildings.

The fourth explosion was from a bomb left on a minibus in the northwestern Risafi area, killing four people and wounding six others, police said.

'Grave challenges'Also in Baghdad, four policemen were killed Wednesday afternoon when gunmen ambushed their patrol south of the city center, police said. Six pedestrians were wounded in the gunfire.

U.S. officials had cited a slight decrease in sectarian killings in Baghdad since the U.S.-Iraqi crackdown was launched Feb. 14. But the past week has seen several spectacular attacks on the capital, including a suicide bombing inside parliament and a powerful blast that collapsed a landmark bridge across the Tigris River.

In Israel, Gates said the military had anticipated that al-Qaida terrorists and other insurgents “would attempt to increase the violence in order to make the plan a failure or to make the people of Iraq believe the plan is a failure.”

“Obviously the level of fatalities today is a horrifying thing. But I think it illustrates another point: These terrorists are killing innocent men, women and children who are Iraqis. They’re killing their countrymen,” he said at a news conference in Tel Aviv with his Israeli counterpart, Amir Peretz.

“We’ve seen both inspiring progress and too much evidence that we still face many grave challenges,” Maj. Gen. William Caldwell, a U.S. military spokesman, told reporters Wednesday. “We’ve always said securing Baghdad would not be easy.”

Militants captured in raidMeanwhile, U.S. troops killed five suspected insurgents and captured 30 others in a raid in Iraq’s western Anbar province, a day after police uncovered 17 decomposing corpses beneath two school yards in the provincial capital.

The U.S. military also announced a discovery made nearly a week earlier — 3,000 gallons of nitric acid hidden in a warehouse in downtown Baghdad. U.S. forces found the acid, a key fertilizer component that can also be used in explosives, during a routine search Thursday, the military said.

Iraqi troops took charge of security Wednesday in the southern province of Maysan, a region that borders Iran and the fourth province to come under full Iraqi security control since the 2003 U.S. invasion.

Security handover to IraqisA ceremony was held in Maysan’s provincial capital of Amarah, 200 miles southeast of Baghdad, and was attended by senior Iraqi and coalition officials including Iraqi National Security Adviser Mowaffak al-Rubaie and the British commander in southern Iraq, Maj. Gen. Jonathan Shaw.

Al-Rubaie said that in order for a timetable to be set for the withdrawal of foreign troops, Iraqi forces and local authorities have to be ready to take over. He was apparently referring to calls by some Sunni Arab groups and al-Sadr’s Shiite followers to set a timetable for a pullout.

“We should work to create these circumstances in all provinces, in order to revert security to Iraqis and end the foreign presence,” said al-Rubaie, who represented Prime Minister Nouri al-Maliki, the commander in chief of Iraq’s armed forces.

Al-Maliki was supposed to attend the ceremony but his trip was canceled without explanation.

The U.S. raid took place early Wednesday near Karmah, a town northeast of Fallujah in Anbar, a vast province west of Baghdad.

U.S. raid yields explosives
American forces raided a group of buildings suspected of being used by militants and found explosives inside one of them, the military said in a statement. A helicopter was called in and dropped precision-guided bombs on the buildings, it said.

The soldiers came under fire and shot back, killing five Iraqis and wounding four others, the statement said. The wounded were taken to a military hospital and remained in U.S. custody. Twenty-six other people were detained as well, the military said.

The bodies found a day earlier at school yards in Ramadi, Anbar’s provincial capital, were discovered after students and teachers returned to the schools a week ago and noticed an increasingly putrid odor and stray dogs digging in the area, police Maj. Laith al-Dulaimi said.

Ramadi had been a stronghold of Sunni insurgents and al-Qaida fighters until recently, when U.S. forces in the region and the Iraqi government successfully negotiated with many local tribal leaders to split them off from the more militant insurgent groups.

The U.S. military also reported that a suspected insurgent was killed and eight captured in two raids north of Baghdad on Wednesday. Some of the suspects were believed linked to al-Qaida in Iraq and to a militant cell that has used chlorine in car bombings, the statement said.

Mortars rain down on MosulIn other violence, two brothers were killed and a policeman was hurt in a gunbattle in downtown Baqouba, 35 miles northeast of Baghdad, police said. The dead were believed to be civilians, caught in the crossfire as police fought unidentified gunmen.

Farther north, 32 mortar shells rained down on Iraqi army checkpoints in two neighborhoods of Mosul, 225 miles northwest of the capital, police said. Six soldiers, a policeman and a pedestrian were injured.

An Iraqi army officer and two soldiers were wounded at dawn in Tal Afar, 47 miles west of Mosul, when gunmen attacked their checkpoint, police said.

In the ethnically mixed city of Kirkuk, 180 miles north of Baghdad, an investigative judge at the city’s criminal court was wounded in a drive-by shooting, police said. Judge Ayad Ali Asaad, a Turkoman, was with his wife and a guard, and all three were wounded.